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Airglow Research With Astronomical Spectra

Applicant Dr. Stefan Noll
Subject Area Atmospheric Science
Term from 2017 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 368912568
 
Final Report Year 2024

Final Report Abstract

The project focused on nighttime emissions in the Earth's upper atmosphere (mainly above 75 km) which originate from chemical reactions that are driven by the destructive impact of hard solar ultraviolet radiation at daytime. This permanent but highly variable chemiluminescence is an important indicator of atmospheric dynamics as caused by different wave-like phenomena as well as the solar influence on the Earth's atmosphere. The radiation is produced by different atoms and molecules in various excited states, which leads to a complex spectrum from the ultraviolet to the infrared. In principle, each emission line shows a characteristic height distribution and variability pattern for a wide range of time scales. Hence, the combined study of many lines with different properties can provide a detailed picture of the ongoing processes at the emission altitudes. In this project, we achieved this goal by the use of high-quality spectra with relatively high resolving power and wavelength coverage from the astronomical echelle spectrographs X-shooter and UVES of the Very Large Telescope at Cerro Paranal in Chile. By the study of archival data that were originally obtained for astronomical purposes, we could cover 15 years with about 10^4 UVES spectra and 10 years with about 10^5 X-shooter spectra, which allowed us to study variations on time scales of hours to years, including the solar activity cycle of about 11 years. In part, the analysis was supplemented by emission profiles from suitable broad-band channels of the SABER radiometer on the TIMED satellite. Moreover, simulations of the chemistry and dynamics were performed for a better understanding of the observations. Based on hundreds of measured lines and the extraction of the unresolved airglow continuum, we obtained the most complete airglow characterisation, so far. In the case of the important OH radical, the results include the systematic description of the excitation structure and its variability, the determination of the quality of temperature estimates from line ratios, and the derivation of effective emission heights of individiual lines. Other highlights are the first variability study of potassium emission and the discovery of a relatively strong pseudo-continuum in the nearinfrared that can convincingly be explained by HO2 emission. There are also results for atomic and molecular oxygen, sodium, FeO, and atomic nitrogen.

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